Bis(triphenylsilyl) chromate is an active catalyst for ethylene polymerization without further treatment or additives. Catalytic activity is markedly increased when the compound is deposited on silica–alumina and is further increased if it is deposited on silica and then treated with an aluminum alkyl. Polymer molecular weight can be controlled by reaction temperature, hydrogen addition, support type, and reducing agent structure to give polymers ranging in melt index from essentially zero to > 100. In the supported catalysts the bis(triphenylsilyl) chromate appears to be bound to the support and to undergo a reduction step either by reaction with ethylene or with aluminum alkyl prior to polymerization. The active site is envisioned as chromium alkyl, bound to the support, with propagation occurring by insertion of the monomer into a CrC bond. Chain termination is by chain transfer to monomer.

synopsisThe effect of various aluminum alkyls at varying concentrations on the rate and stereospecificity of propylene polymerizations with titanium trichloride was examined. It was concluded that diakylaluminum halides were merely chemisorbed on the surface of the titanium trichloride while the trialkylaluminums reacted more extensively with the surface. I n the case of diethylaluminum chloride standard chemisorption kinetics were observed. The rate of polymerization was also found to be a function of solvent, with certain aromatic solvents causing significant rate increases. With diethylaluminum chloride essentially no-termination occurs; the polymers are "living polymers."